Satoshi Yasui is the kind of designer who can riff on any product--including socks. Not just any socks, but comfy ones with a 90-degree heel, knit for a perfect fit by Czech grandmothers, that he and his 15-member design team at Muji transformed into one of the Japanese retailer's roughly 7,000 products. "They don't fall off like regular socks, which are usually manufactured with a 120-degree angle," explains Yasui, lifting one cuff of his black jeans to reveal a pair. Yasui--who has been with Muji since the Seiyu supermarket chain created it as a private brand in 1980--says that sometimes the...